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Dark Stars: a Review 2 Dark Stars: A Review Katherine Freese1,2,3, Tanja Rindler-Daller3,4, Douglas Spolyar2 and Monica Valluri5 1 Nordita (Nordic Institute for Theoretical Physics), KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, SE-106 91 Stockholm, Sweden 2The Oskar Klein Center for Cosmoparticle Physics, AlbaNova University Center, University of Stockholm, 10691 Stockholm, Sweden 3 Department of Physics and Michigan Center for Theoretical Physics, University of Michigan, 450 Church St., Ann Arbor, MI 48109, USA 4 Institute for Astrophysics, Universit¨atssternwarte Wien, University of Vienna, T¨urkenschanzstr. 17, A-1180 Wien, Austria 5 Department of Astronomy, University of Michigan, 1085 South University Ave., Ann Arbor, MI 48109, USA arXiv:1501.02394v2 [astro-ph.CO] 4 Mar 2016 Dark Stars: A Review 2 Abstract. Dark Stars are stellar objects made (almost entirely) of hydrogen and helium, but powered by the heat from Dark Matter annihilation, rather than by fusion. They are in hydrostatic and thermal equilibrium, but with an unusual power source. Weakly Interacting Massive Particles (WIMPs), among the best candidates for dark matter, can be their own antimatter and can annihilate inside the star, thereby providing a heat source. Although dark matter constitutes only <∼ 0.1% of the stellar mass, this amount is sufficient to power the star for millions to billions of years. Thus, the first phase of stellar evolution in the history of the Universe may have been dark stars. We review how dark stars come into existence, how they grow as long as dark matter fuel persists, and their stellar structure and evolution. The studies were done in two different ways, first assuming polytropic interiors and more recently using the MESA stellar evolution code; the basic results are the same. Dark stars are giant, puffy (∼ 10 AU) and cool (surface temperatures ∼10,000 K) objects. We follow the evolution of dark stars from their inception at ∼ 1M⊙ as they accrete mass from their surroundings to become 6 10 supermassive stars, some even reaching masses > 10 M⊙ and luminosities > 10 L⊙, making them detectable with the upcoming James Webb Space Telescope. Once the dark matter runs out and the dark star dies, it may collapse to a black hole; thus dark stars may provide seeds for the supermassive black holes observed throughout the Universe and at early times. Other sites for dark star formation may exist in the Universe today in regions of high dark matter density such as the centers of galaxies. The current review briefly discusses dark stars existing today, but focuses on the early generation of dark stars. PACS numbers: 98.80.-k; 95.35.+d; 97.10.-q 1. Introduction Dark Stars (DSs) are stellar objects powered by the heat from Dark Matter (DM) annihilation. We will focus on the DSs that may have been the first stars to form in the history of the Universe, and briefly discuss DSs that may exist today. The first stars formed when the Universe was roughly 200 million years old, at redshifts z ∼ 10 − 50. We will show that these first stars, which form in a dark matter rich environment, may have been Dark Stars, powered by dark matter heating rather than by fusion for millions to billions of years. Only after the dark matter fuel was exhausted could fusion take over as the power source inside stars‡. Weakly Interacting Massive Particles (WIMPs) are thought to be among the best motivated dark matter candidates. Many WIMP candidates are their own antiparticles, and if they are initially in thermal equilibrium in the early Universe, they annihilate with one another so that a predictable number of them remain today. Once the annihilation rate drops below the Hubble expansion rate, the abundance of WIMPs freezes out. The relic density of these particles is approximately [1, 2] 3 × 10−27cm3/sec Ω h2 ≃ , (1) χ hσvi ‡ The largest supermassive dark stars may bypass fusion altogether and collapse directly to black holes. Dark Stars: A Review 3 where Ωχ is the fraction of the energy density in the Universe today in the form of WIMPs and h is the Hubble constant in units of 100km/s/Mpc. With the simple assumption that the annihilation cross section hσvi is determined by weak interaction strength, then WIMPs automatically produce roughly the correct dark matter density today, ∼ 25% of the total content of the Universe [3, 4]. This coincidence is known as “the WIMP miracle” and is the reason why WIMPs are taken so seriously as DM candidates. The Universe as a whole consists of roughly 5% baryonic material, 25% dark matter, and 70% dark energy§. There is a second reason for the interest in WIMPs as dark matter candidates: WIMPs automatically exist in particle theories designed to solve problems that have nothing to do with dark matter. Supersymmetric (SUSY) extensions of the standard model of particle physics predict the existence of new partners for every particle in the standard model and, given R-parity, the lightest of these would be dark matter candidates. In particular, an excellent WIMP candidate is the lightest neutralino in the Minimal Supersymmetric Standard Model. Models of universal extra dimensions may also have WIMP dark matter candidates in the theories (e.g. Kaluza- Klein particles) k. The same annihilation process that took place throughout the early Universe continues in those locations where the dark matter density is sufficiently high for WIMPs to encounter one another and annihilate. The first stars to form in the Universe are a natural place to look for significant amounts of dark matter annihilation, because they formed “at the right place and the right time”: they formed early (when the Universe was still substantially denser than it is today), and at the high density centers of dark matter halos. The formation of large-scale structures in the Universe – the galaxies and galaxy clusters – took place via a process known as hierarchical clustering. As the dominant component of the mass in the Universe, dark matter drove the dynamics of this formation of structure. Small (sub-Earth-sized) clumps formed first; then these merged together to make larger structures; and eventually these merged yet further to produce the galaxies and clusters we see today. These clumps of various sizes, known as “dark matter halos,” are spheroidal (prolate or triaxial) objects containing 85% dark matter and 15% atomic matter. The remainder of the Universe, the dark energy, does not respond to the attractive force of gravity and instead produces an accelerated expansion of the Universe; dark energy played no role in the formation of the first stars. At the time of the formation of the first stars, the atomic matter in the Universe consisted only of hydrogen, helium, and a smattering of heavier elements (Li, B, Be) – the products of primordial nucleosynthesis that took place three minutes after the Big Bang. All the other more complex elements were only able to form later, as the products of fusion in stars. § There is some disagreement between the best fit values of the PLANCK and WMAP satellites [3, 4], but the numbers we quote here are roughly correct [3, 4]. k DM is not limited to self-conjugate Majorana states, but these are traditionally the most studied possibilities. Dark Stars: A Review 4 6 Once dark matter halos about a million times as massive as the Sun (10 M⊙), known as ’minihalos’¶, were able to form, the conditions were ripe for the formation of the first stars, known as Population III stars. The virial temperatures of minihalos led to molecular hydrogen cooling that allowed a protostellar cloud to start to collapse towards the center of the halo. Reviews of the standard picture of the formation of the first stars, without taking into account the role of dark matter, can be found in Ref. [6, 7, 8, 9, 10, 11]. It was the idea of some of the authors of this review to ask, what is the effect of the DM on these first stars? We studied the behavior of WIMPs in the first stars, and found that they can radically alter the stars’ evolution [12]. The annihilation products of the dark matter inside the star can be trapped and deposit enough energy to heat the proto-star and prevent it from further collapse. A new stellar phase results, a “Dark Star”, powered by DM annihilation as long as there is DM fuel, for possibly millions to billions of years. The DM – while only a negligible fraction of the star’s mass – provides the key power source for the star through DM heating. Note that the term ‘dark’ refers to the power source, not the material or the appearance of the star. Early DSs are stars made primarily of hydrogen and helium with a smattering of dark matter; typically less than 0.1% of the mass consists of DM. Yet, DSs shine due to DM heating. In the past few years, we have done extensive studies of the stellar structure and evolution of DSs. Dark stars are born with masses ∼ 1M⊙ and then grow to much larger masses. They are giant, puffy (10 AU), and cool (surface temperatures ∼ 10, 000K) objects [13]. Since the DSs reside in a large reservoir of baryons (15% of the total halo mass), the baryons can start to accrete onto the DSs. Our work [13, 14, 15, 16] followed the evolution of DSs from their inception at ∼ 1M⊙, as they accreted mass from their 7 surroundings to become supermassive stars, possibly as large as 10 M⊙. We now have used two different approaches in studying the evolution of dark stars. In our initial studies, we assumed that the star can be described as a polytrope with the relationship between pressure P and density ρ at a given radius determined by the polytropic index n, P = Kρ1+1/n, (2) where the “constant” K is determined, once we know the total mass and radius [17].
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